Initial loading core

ABSTRACT

Several unit loading patterns are arranged in the central area of an initial core to which the present invention is applied. The unit loading pattern is composed of one square-shaped unit cell and four cross-shaped control rods 3 which surround the unit cell. The unit cell is composed of one low enrichment fuel assembly 7, two high enrichment fuel assemblies 8 and one high enrichment fuel assembly 9. The low enrichment fuel assemblies 7 of each unit loading pattern adjoin each other and are arranged to constitute the first control cell 2a being square-shaped. The high enrichment fuel assemblies 9, obliquely adjoining the low enrichment fuel assembly 7 in each unit loading pattern, adjoin each other and are arranged to constitute the second control cell 2b being square-shaped. The high enrichment fuel assembly 9 constituting the unit loading pattern is divided into a control rod side area and an anticontrol rod side area by a diagonal line L1, and the number of Gd fuel rods in the control rod side area is 2 or more than the number in the anticontrol rod side area. By providing this structure, the increase of the local peaking factor on the control rod side can be suppressed and the thermal margin can be sufficiently secured, even if the control rod 3 of the second control cell 2b is extracted after the second operation cycle.

BACKGROUND OF THE INVENTION

The present invention relates to an initial core of a boiling waterreactor (BWR).

It is necessary to increase the average enrichment of an initial core toraise the discharge exposure of the initial core. Therefore, in aninitial core, which is loaded with a plurality of fuel assemblies havinga different average enrichment, the difference in the nuclearcharacteristics between a high enrichment fuel assembly having a highaverage enrichment and a low enrichment fuel assembly having a lowaverage enrichment becomes large.

In case a high enrichment fuel assembly and a low enrichment fuelassembly adjoin each other, a thermal neutron is caused to flow from thelow enrichment fuel assembly with a high thermal neutron flux to thehigh enrichment fuel with a low thermal neutron flux. Therefore, becausethe output of the fuel rods of the high enrichment fuel assemblyincreases, and the MLHGR (Maximum Linear Power Heat Generation Ratio)and the MCPR (Minimum Critical Power Ratio) in the beginning of burnupbecome severe, an improvement in the thermal margin becomes a problem.

To improve the thermal margin, fuel rods containing gadolinia(hereinafter called a "Gd fuel rod") are arranged symmetrically in thecross section of a conventional fuel assembly. In case the averageenrichment of an initial core is heightened for high burnup, controlrods are inserted in the core for a long time, and then they areextracted to suppress excess reactivity.

Therefore, the output of one side near to the control rod is smallerthan that of the other side far from the control rod in the crosssection of a fuel assembly loaded into a control cell. Burnup of fuelrods on the one side near to the control rod is delayed. This is calledcontrol rod history effect. Due to the influence of this control rodhistory effect, the thermal margin could not be sufficiently secured ina conventional initial core. Particularly, in a case where the highenrichment fuel was loaded into the control cell in a second operationcycle, the thermal margin was severe.

SUMMARY OF THE INVENTION

It is an object of the present invention to secure the thermal margin inan initial core that increases the average enrichment for a high burnupand uses a high enrichment fuel assembly for the control cell in thesecond operation cycle.

In accordance with the present invention, to achieve the above object,there is provided an initial core comprising a plurality of fuelassemblies having different average enrichment and a plurality ofcontrol rods, each fuel assembly having a square-shaped cross sectionand each control rod having a cross-shaped cross section, wherein a unitcell of a square-form is composed of one first fuel assembly with thelowest average enrichment and three second fuel assemblies with higheraverage enrichment than the first fuel assembly. A plurality of unitloading patterns are arranged in a central area of the initial core,each unit loading pattern being composed of one unit cell and fourcontrol rods arranged at four corners of said one unit cell, and thesecond fuel assembly obliquely adjoining the first fuel assembly in eachunit loading pattern is divided into one side area near to the controlrod and the other side area far from the control rod by a diagonal line,the number of fuel rods containing gadolinia in said one side area being2 or more than the number in said other side area.

To examine an effect according to the present invention, the localpeaking factor of the high enrichment fuel assembly that composes thecontrol cell in the second operation cycle was analytically analyzed.

FIG. 5 shows a unit loading pattern of a comparative example. Thiscomparative example includes one low enrichment fuel assembly 7, twohigh enrichment fuel assemblies 8 and one high enrichment fuel assembly9a like that of FIG. 2 to be mentioned later. Low enrichment fuelassembly 7 is equivalent to the first fuel assembly and high enrichmentfuel assemblies 8 and 9a are equivalent to the second fuel assemblies.In FIG. 5, high enrichment fuel assembly 9a is divided into a controlrod side area and an anticontrol rod side area by a diagonal line L1.The number of Gd fuel rods 10 in the control rod side area is 4 and thenumber of Gd fuel rods 10 in the anticontrol rod side area is 10.

By use of the comparative example of FIG. 5, the local peaking factor ina cross section perpendicular to an axial direction of the highenrichment fuel assembly 9a in exposure of 20 GWd/t was analyzed for two2 cases. The first case is a first comparative example wherein controlrod 3 is not inserted in a core until an exposure of 20 GWd/t(equivalent to end of the second operation cycle) is reached. The secondcase is a second comparative example wherein control rod 3 is notinserted in the core until the exposure of 10 GWd/t (equivalent to endof the first operation cycle) is reached, then the control rod 3 isinserted in the core until the exposure of 20 GWd/t is reached, andfinally the control rod 3 is extracted from the core when the exposureof 20 GWd/t has been reached.

Analytical results of the first comparative example are shown in FIG. 6and analytical results of the second comparative example are shown inFIG. 7. The above-mentioned cross section perpendicular to the axialdirection is a cross section of 2/24-10/24 from the lower end of fuelactive length of the fuel assembly. As shown in FIG. 6 and FIG. 7, thelocal peaking factor on the control rod side of the second comparativeexample becomes larger than that of the first comparative example due tothe influence of the control rod history effect. As shown in FIG. 7, thelocal peaking factor of a corner on the control rod side becomesmaximum.

Next, FIG. 8 shows analytical results of the local peaking factor in thecross section perpendicular to the axial direction of the presentinvention shown in FIG. 2 to be mentioned later at the exposure of 20GWd/t, when the control rod of the present invention shown in FIG. 2 isoperated in the same way as the second comparative example. Highenrichment fuel assembly 9 of FIG. 2 is divided into a control rod sidearea and an anticontrol rod side area by the diagonal line L1, thenumber of the Gd fuel rods 10 in the control rod side area is 10 and thenumber of the Gd fuel rods 10 in the anticontrol rod side area is 4. Thedifference in the number of the Gd fuel rods (hereinafter called "Gdfuel rod difference") between these two areas is 6. The distribution ofGd fuel rods 10 in FIG. 2 is reversed relative to that in thecomparative example of FIG. 5.

From FIG. 8, it is seen that an increase in the local peaking factor onthe control rod side in accordance with the present invention can besuppressed relative to that in the second comparative example. That is,by arranging a greater number of Gd fuel rods 10 on the control rod sideas compared to the anticontrol rod side in the high enrichment fuelassembly 9, when the control rod 3 is extracted from a control cellcomposed of the high enrichment fuel assembly 9 after the secondoperation cycle, the increase of the local peaking factor on the controlrod side can be suppressed and the thermal margin can be secured.Especially, it is effective to arrange a Gd fuel rod 10 in the corner ofthe second layer from the outside on the control rod side of the highenrichment fuel assembly 9 in order to suppress the local peaking factorat the corner on the control rod side which becomes maximum.

Next, the relation between the Gd fuel rod difference and the abovementioned suppression effect of the local peaking factor will beexplained with reference to FIG. 17.

FIG. 17 shows the Gd fuel rod difference, which is defined as thedifference (n1-n2) between the number n1 of the Gd fuel rods 10 in thecontrol rod side area and the number n2 of the Gd fuel rods 10 in theanticontrol rod side area, along the horizontal axis, versus the maximumvalue of fuel rod output along the vertical axis, when the position ofthe Gd fuel rods 10 in the high enrichment fuel assembly 9 is changed.In case a Gd fuel rod exists on the diagonal line L1, 0.5 is added tothe number of Gd fuel rods in the control rod side area and theanticontrol rod side area, respectively.

As shown in FIG. 17, the maximum value of fuel rod output tends to beginto decrease from a value of the Gd fuel rod difference of about 1 andbecomes almost saturated at a value of the Gd fuel rod difference over3. From this tendency, it is established that the maximum value of fuelrod output can be effectively reduced by making the Gd fuel roddifference 2 or more. Therefore, an increase in the local peaking factoron the control rod side can be suppressed and the thermal margin can besecured by making the Gd fuel rod difference 2 or more. Furthermore, themore desirable range of Gd fuel rod difference is 3 or more in which theabove mentioned saturation tendency appears.

FIG. 17 does not show an upper limit of the Gd fuel rod difference. But,there is an upper limit of the Gd fuel rod difference and this upperlimit is about 1/4 of the number of all fuel rods in the fuel assembly.This upper limit is determined by the following factors. The Gd fuel rodis arranged at a position excluding outermost positions of the fuelassembly. Therefore, it is impossible to arrange a larger number of Gdfuel rods in the control rod side area than the fuel assembly shown inFIG. 18. In case of FIG. 18, the Gd fuel rod difference is 19 and isabout 1/4 of 74 which is the number of all fuel rods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional diagram showing one-fourth of a firstexample of an initial core according to the present invention.

FIG. 2 is a cross sectional view of the first example of a unit loadingpattern according to the present invention.

FIG. 3 is a diagram showing the distribution of enrichment and gadoliniain the axial direction of the high enrichment fuel of FIG. 2.

FIG. 4 is a partially broken-away perspective view of a fuel assemblyaccording to the present invention.

FIG. 5 is a cross sectional view of the unit loading pattern of acomparative example.

FIG. 6 is a diagram showing an analytical result of local peaking factorof a first comparative example.

FIG. 7 is a diagram showing an analytical result of local peaking factorof a second comparative example.

FIG. 8 is a diagram showing an analytical result of local peaking factoraccording to the present invention.

FIG. 9 is a cross sectional view of a second example of a unit loadingpattern according to the present invention.

FIG. 10 is a cross sectional view of a third example of a unit loadingpattern according to the present invention.

FIG. 11 is a cross sectional view of a fourth example of a unit loadingpattern according to the present invention.

FIG. 12 is a cross sectional diagram showing one-fourth of a secondexample of an initial core according to the present invention.

FIG. 13 is a cross sectional view of a fifth example of a unit loadingpattern according to the present invention.

FIG. 14 is a cross sectional view of a sixth example of a unit loadingpattern according to the present invention.

FIG. 15 is a cross sectional view of a seventh example of a unit loadingpattern according to the present invention.

FIG. 16 is a cross sectional view of an eighth example of a unit loadingpattern according to the present invention.

FIG. 17 is a graph of the relationship between the Gd fuel roddifference and local peaking factor.

FIG. 18 is a diagram of the upper limit of the Gd fuel rod difference.

PREFERRED EMBODIMENT OF THE PRESENT INVENTION

Various embodiments of the present invention now will be described withreference to the drawings.

FIG. 1 is a cross sectional diagram showing one-fourth of a firstexample of an initial core according to the present invention. This coreis composed of 872 fuel assemblies, which include 208 low enrichmentfuel assemblies 7, 304 high enrichment fuel assemblies 8 and 360 highenrichment fuel assemblies 9. An average enrichment of the lowenrichment fuel assemblies 7 is lower than that of the high enrichmentfuel assemblies 8. The average enrichment of the high enrichment fuelassemblies 8 and 9 is equal.

Several unit loading patterns, as shown in FIG. 2, are arranged in acentral area, which is an inner area beginning at the second layer fromthe outermost periphery of the core. FIG. 2 shows a cross sectional viewof the first example of a unit loading pattern according to the presentinvention. This unit loading pattern is composed of one square-shapedunit cell and four cross-shaped control rods 3 which surround the unitcell. The unit cell is composed of one low enrichment fuel assembly 7,two high enrichment fuel assemblies 8 and one high enrichment fuelassembly 9. In the core of FIG. 1, 128 unit loading patterns are loaded.

In the central area of the core, the low enrichment fuel assembly 7 ofeach unit loading pattern adjoins each other fuel assembly thereof andis arranged to constitute the first control cell 2a of square-shape. Thehigh enrichment fuel assembly 9, obliquely adjoining the low enrichmentfuel assembly 7 in each unit loading pattern, adjoins each other fuelassembly therein and is arranged to constitute the second control cell2b of square-shape. That is, four unit loading patterns, each of whichinclude a first control cell 2a and a second control cell 2bsymmetrically arranged relative to the center of the first control cell2a and the second control cell 2b, respectively, are provided.

The core of FIG. 1 has 29 first control cells 2a and 32 second controlcells 2b. The control rod 3 is inserted in the first control cell 2a inthe first operation cycle and inserted in the second control cell 2bmainly in the second operation cycle.

Each fuel assembly constituting a unit loading pattern has fuel rods 6arranged in a square-form of 9 columns and 9 rows (9×9) and twolarge-diameter water rods 5 in which water flows. Two water rods 5 arearranged in an area in which seven fuel rods can be arranged.

As shown in a partially broken-away perspective view of FIG. 4, anactual fuel assembly is composed of an upper tie plate 4a, a lower tieplate 4d, a channel fastener 4b, spacers 4c, water rods 5 (notillustrated), fuel rods 6, channel box 4 and so on. The channel fastener4b installed in a corner of the upper tie plate 4a is a means for fixingthe fuel assembly to the control rod 3. Therefore, in the fuel assemblyitself, the control rod side area is equivalent to an area on a sidewhere the channel fastener 4b exists.

The number of Gd fuel rods 10 in the high enrichment fuel assembly 9constituting the unit loading pattern of FIG. 2 is 14. Ten Gd fuel rods10 are arranged in the control rod side area and four Gd fuel rods 10are arranged in the anticontrol rod side area. The difference in thenumber of Gd fuel rods (Gd fuel rod difference) between these two areasis 6. The number of the Gd fuel rods 10 in the high enrichment fuelassembly 8 is 15. Five Gd fuel rods 10 are arranged in the control rodside area and ten Gd fuel rods 10 are arranged in the anticontrol rodside area.

FIG. 3 shows the distribution of the enrichment and gadolinia in theaxial direction of the high enrichment fuel assembly 9 of FIG. 2. Thehigh enrichment fuel assembly 9 is composed of fuel rods A˜D containinguranium fuel and no gadolinia in the over-all length of a fuel activelength, Gd fuel rod G containing uranium fuel in the overall length ofthe fuel active length and gadolinia in a range of 1/24-22/24 from thelower end of the fuel active length, short-length fuel rod E containinguranium fuel and no gadolinia in a range of 1/24-15/24 from the lowerend of the fuel active length, and Gd fuel rod F containing uranium fuelin the range of 1/24-15/24 and gadolinia in a range of 1/24-8/24 fromthe lower end of the fuel active length. The number of each of the fuelrods provided is as shown in FIG. 3.

The fuel rods A-D and the Gd fuel rod G are loaded with natural uranium(enrichment of 0.711 wt %) in both a lower end region of 0/24-1/24 andan upper end region of 22/24-24/24 from the lower end of the fuel activelength. The fuel rods A-D are loaded with uranium fuel of 4.9, 4.4, 4.0and 3.1 wt %, respectively, in the range of 1/24-22/24 from the lowerend of the fuel active length. The Gd fuel rod G is loaded with uraniumfuel of 4.4 wt % and gadolinia of 7.5 wt % in the range of 1/24-22/24from the lower end of the fuel active length. The Gd fuel rod F isloaded with uranium fuel of 4.4 wt % and gadolinia of 7.5 wt % in therange of 1/24-8/24 and uranium fuel of 4.4 wt % in a range of 8/24-15/24from the lower end of the fuel active length. The short-length fuel rodE is loaded with uranium fuel of 4.9 wt % in the range of 1/24-15/24from the lower end of the fuel active length.

By a combination of the fuel rods shown in FIG. 3, the high enrichmentfuel 9 makes an average enrichment of a cross section perpendicular tothe axial direction about 4.59 wt % in the range of 1/24-15/24 and makesthe average enrichment of the cross section about 4.56 wt % in the rangeof 15/24-22/24 from the lower end of the fuel active length.

On the other hand, the low enrichment fuel assembly 7 of FIG. 2 isloaded with no gadolinia and the natural uranium in both the lower endregion and the upper end region. The average enrichment of the lowenrichment fuel assembly 7 is lower than that of the high enrichmentfuel assemblies 8 and 9.

According to this example, as shown in FIG. 17, by making the Gd fuelrod difference of the high enrichment fuel assembly 9 six so as to belarger than two, the thermal margin can be sufficiently secured becausean increase of the local peaking factor on the control rod side can besuppressed even if the control rod 3 of the second control cell 2b isextracted after the second operation cycle.

Especially, by arranging the Gd fuel rod 10 in the corner of the secondlayer from the outside on the control rod side of the high enrichmentfuel assembly 9, the local peaking factor of the corner on the controlrod side can be effectively suppressed. Furthermore, the channel peakingfactor in the core can be reduced because the loading pattern of thefuel assembly in the central area of the core, which has a relativelyhigh output, is almost non-uniform. This also contributes to a securingof the thermal margin.

As the high enrichment fuel assembly 8 of this example, an uranium fuelassembly only containing uranium fuel (hereinafter called "uraniumfuel") or a MOX fuel assembly containing plutonium fuel (hereinaftercalled "MOX fuel") can be used. Furthermore, the uranium fuel and theMOX fuel can be used for the two high enrichment fuel assemblies 8. Inthis case, the average enrichment of both uranium and plutonium has onlyto be higher than that of the low enrichment fuel assembly 7.

Next, a second example of a unit loading pattern according to thepresent invention will be explained by reference to FIG. 9. FIG. 9 showsa cross sectional view of this second example. The number of the Gd fuelrods 10 in the high enrichment fuel assembly 9 constituting this unitloading pattern is 16. Eleven Gd fuel rods 10 are arranged in thecontrol rod side area and five Gd fuel rods 10 are arranged in theanticontrol rod side area. The Gd fuel rod difference between these twoareas is 6. The number of the Gd fuel rods 10 in each of the highenrichment fuel assemblies 8 is 12. Six Gd fuel rods 10 are arranged inthe control rod side area and six Gd fuel rods 10 are arranged in theanticontrol rod side area.

In this example, by making the Gd fuel rod difference of the highenrichment fuel assembly 9 six so as to be larger than two, the increasein the local peaking factor on the control rod side can be suppressedand the thermal margin can be sufficiently secured similar to the firstexample of FIG. 2, even if the control rod 3 of the second control cell2b is extracted after the second operation cycle.

Like the first example and the second example, whether the Gd fuel roddifference of the high enrichment fuel assemblies 8 in the unit loadingpattern is large or small, by making the Gd fuel rod difference of thehigh enrichment fuel 9 two or more, the increase of the local peakingfactor by the control rod history effect can be suppressed.

Next, a third example of a unit loading pattern according to the presentinvention will be explained with reference to FIG. 10. FIG. 10 shows across sectional view of this third example. The number of the Gd fuelrods 10 in the high enrichment fuel assembly 9 constituting this unitloading pattern is 12. Seven Gd fuel rods 10 are arranged in the controlrod side area and five Gd fuel rods 10 are arranged in the anticontrolrod side area. The Gd fuel rod difference between these two areas is 2.The number of the Gd fuel rods 10 in the high enrichment fuel assemblies8 is 13. Three Gd fuel rods 10 are arranged in the control rod side areaand ten Gd fuel rods 10 are arranged in the anticontrol rod side area.

In this example, by making the Gd fuel rod difference of the highenrichment fuel assembly 9 equal to two, the increase of the localpeaking factor on the control rod side can be suppressed and the thermalmargin can be sufficiently secured similar to the first example of FIG.2, even if the control rod 3 of the second control cell 2b is extractedafter the second operation cycle.

Next, a fourth example of a unit loading pattern according to thepresent invention will be explained with reference to FIG. 11. FIG. 11shows a cross sectional view of this fourth example. In this example,the low enrichment fuel assembly 7, the high enrichment fuel assemblies8 and 9, and a middle enrichment fuel assembly 14 are used. The averageenrichment of the middle enrichment fuel assembly 14 is lower than thatof the high enrichment fuel assemblies 8 and 9, and is higher than thatof low enrichment fuel assembly 7.

The number of the Gd fuel rods 10 in the high enrichment fuel assembly 9is 12. Ten Gd fuel rods 10 are arranged in the control rod side area andtwo Gd fuel rods 10 are arranged in the anticontrol rod side area. TheGd fuel rod difference between these two areas is 8. The number of theGd fuel rods 10 in the high enrichment fuel assembly 8 is 13. Three Gdfuel rods 10 are arranged in the control rod side area and ten Gd fuelrods 10 are arranged in the anticontrol rod side area. The number of theGd fuel rods 10 in the middle enrichment fuel assembly 14 is 5. Two Gdfuel rods 10 are arranged in the control rod side area and three Gd fuelrods 10 are arranged in the anticontrol rod side area.

Even when the middle enrichment fuel assembly 14 is provided in the unitloading pattern like this example, by making the Gd fuel rod differenceof the high enrichment fuel assembly 9 eight so as to be larger thantwo, the increase of the local peaking factor on the control rod sidecan be suppressed and the thermal margin can be sufficiently securedsimilar to the first example of FIG. 2, even if the control rod 3 of thesecond control cell 2b is extracted after the second operation cycle.

FIG. 12 is a cross sectional diagram showing one-fourth of the secondexample of an initial core according to the present invention in whichthe unit loading pattern of FIG. 11 is arranged in the central area ofthe core. In this core, the first control cell 2a, the second controlcell 2b, the low enrichment fuel 7 and the high enrichment fuel assembly9 are the same as the first example of FIG. 1. A difference from thefirst example is that part of the high enrichment fuel assemblies 8 arereplaced with the middle enrichment fuel assemblies 14, such that 172high enrichment fuel assemblies 8 and 132 middle enrichment fuelassemblies 14 are loaded into the central area of the core.

Next, a fifth example of a unit loading pattern according to the presentinvention will be explained with reference to FIG. 13. FIG. 13 shows across sectional view of this fifth example. Three high enrichment fuelassemblies 8 and 9 of the four fuel assemblies constituting the unitloading pattern of this example are the same as the second example ofFIG. 9, and the low enrichment fuel assembly 7 is different from thesecond example. The low enrichment fuel assembly 7 of this example hasfuel rods 6 arranged in a square lattice-form of 8 columns and 8 rows(8×8) and one large-diameter water rod 5 arranged in an area in whichfour fuel rods can be arranged.

In this example, by making the Gd fuel rod difference of the highenrichment fuel assembly 9 six so as to be larger than two, the increasein the local peaking factor on the control rod side can be suppressedand the thermal margin can be sufficiently secured similar to the secondexample of FIG. 9, even if the control rod 3 of the second control cell2b is extracted after the second operation cycle.

Even if the unit loading pattern is composed of fuel assemblies whichhave a different configuration like this example, by making the Gd fuelrod difference of the high enrichment fuel assembly 9 two or more, theincrease of the local peaking factor by the control rod history effectcan be suppressed.

Next, a sixth example of a unit loading pattern according to the presentinvention will be explained with reference to FIG. 14. FIG. 14 shows across sectional view of this sixth example. Four fuel assembliesconstituting the unit loading pattern of this example have the sameshape as the low enrichment fuel assembly 7 shown in FIG. 13. In FIG.14, the MOX fuel assembly 15 contains plutonium from the time of beinginitially loaded in the core. The MOX fuel assembly 15 has a fuel rod 16containing no gadolinia and a Gd fuel rod 17 containing gadolinia. Partof the fuel rods 16 and part of the Gd fuel rods 17 contain plutonium.Of course, it is possible that the Gd fuel rods 17 contain no plutonium.

The number of the Gd fuel rods 10 in the high enrichment fuel assembly 9is 13. Eight Gd fuel rods 10 are arranged in the control rod side areaand five Gd fuel rods 10 are arranged in the anticontrol rod side area.The Gd fuel rod difference between these two areas is 3. The number ofthe Gd fuel rods 17 in the MOX fuel assembly 15 is 13. Five Gd fuel rods10 are arranged in the control rod side area and eight Gd fuel rods 10are arranged in the anticontrol rod side area.

In case the shape of the fuel assembly changes, as in this example, atleast by making the Gd fuel rod difference of the high enrichment fuelassembly 9 two or more, the increase of the local peaking factor on thecontrol rod side can be suppressed and the thermal margin can besufficiently secured, even if the control rod 3 of the second controlcell 2b is extracted after the second operation cycle.

In this example, two MOX fuel assemblies 15 containing plutonium arearranged to adjoin each other obliquely, and the low enrichment fuelassembly 7 and the high enrichment fuel assembly 9 are arranged toadjoin each other obliquely. But, it is possible to replace one of theMOX fuel assemblies 15 with a high enrichment fuel assembly 8.

Next, a seventh example of a unit loading pattern according to thepresent invention will be explained with reference to FIG. 15. FIG. 15shows a cross sectional view of this seventh example. Four fuelassemblies constituting the unit loading pattern of this example havefuel rods arranged in a square lattice-form of 9×9 like the firstexample of FIG. 2 and one water box 13. The water box 13 occupies anarea in which 9 fuel rods can be arranged. The number of the fuel rodsin the fuel assembly is 72.

The number of the Gd fuel rods 10 in the high enrichment fuel assembly 9of this example is 12. Nine Gd fuel rods 10 are arranged in the controlrod side area and three Gd fuel rods 10 are arranged in the anticontrolrod side area. The Gd fuel rod difference between these two areas is 6.The number of the Gd fuel rods 10 in the high enrichment fuel assembly 8is 13. Ten Gd fuel rods 10 are arranged in the control rod side area andthree Gd fuel rods 10 are arranged in the anticontrol rod side area.

In this example, by making the Gd fuel rod difference of the highenrichment fuel assembly 9 six so as to be larger than two, the increaseof the local peaking factor on the control rod side can be suppressedand the thermal margin can be sufficiently secured similar to the firstexample of FIG. 2, even if the control rod 3 of the second control cell2b is extracted after the second operation cycle.

Next, an eighth example of a unit loading pattern according to thepresent invention will be explained with reference to FIG. 16. FIG. 16shows a cross sectional view of this eighth example. The high enrichmentfuel assemblies 8 and 9 constituting the unit loading pattern of thisexample are the same as those in the first example of FIG. 2 and the lowenrichment fuel assembly 7 is different. The low enrichment fuelassembly 7 has the water rods 5 shifted in position from the center ofthe fuel assembly.

In case the water rods are shifted from the center of the fuel assemblyas in this example, by making the Gd fuel rod difference of the highenrichment fuel assembly 9 two or more, the increase of the localpeaking factor on the control rod side can be suppressed and the thermalmargin can be sufficiently secured, even if the control rod 3 of thesecond control cell 2b is extracted after the second operation cycle.

What is claimed is:
 1. An initial core comprising a plurality of fuelassemblies having different average enrichment and a plurality ofcontrol rods, each fuel assembly having a square-shaped cross sectionand each control rod having a cross-shaped cross section,wherein a unitcell of a square-form is composed of one first fuel assembly with thelowest average enrichment and three second fuel assemblies with higheraverage enrichment than the first fuel assembly, a plurality of unitloading patterns arranged in a central area of the initial core, eachunit loading pattern being composed of one unit cell and four controlrods arranged at four corners of said one unit cell, the second fuelassembly obliquely adjoining the first fuel assembly in each unitloading pattern is divided into one side area near to the control rodand another side area far from the control rod by a diagonal line, anumber of fuel rods containing gadolinia in said one side area being 2or more than the number of fuel rods containing gadolinia in saidanother side area of the second fuel assembly obliquely adjoining thefirst fuel assembly.
 2. An initial core claimed in claim 1, wherein saidunit loading patterns are arranged to constitute control cells of asquare-form, each control cell being composed of four fuel assemblessaid second fuel assemblies obliquely adjoining the first fuel assemblyadjoined each other.
 3. An initial core claimed in claim 1 or 2, wherein9 difference of the number of fuel rods containing gadolinia in said oneside area and that in said another side area is in the range of between3 and
 19. 4. An initial core claimed in any one of claims 1 or 2,wherein said second fuel assembly obliquely adjoining the first fuelassembly in each unit loading pattern has a fuel rod containinggadolinia at the corner of the second layer from the outermost peripheryin said one side area.
 5. An initial core claimed in any one of claims 1or 2, wherein at least one of two second fuel assemblies except for saidsecond fuel assembly obliquely adjoining the first fuel assembly in eachunit loading pattern contains plutonium.
 6. An initial core claimed inany one of claims 1 or 2, wherein at least the second fuel assembly hasfuel rods arranged in the lattice-pattern of 9 columns and 9 rows andtwo large-diameter water rods arranged in an area of a size to arrangeseven fuel rods.
 7. An initial core claimed in any one of claims 1 or 2,wherein at least the second fuel assembly has fuel rods arranged in thelattice-pattern of 9 columns and 9 rows and one water box arranged in anarea of a size to arrange nine fuel rods.
 8. An initial core comprisinga plurality of first fuel assemblies containing plutonium, a pluralityof second fuel assemblies containing no plutonium and a plurality ofcontrol rods, each fuel assembly having a square-shaped cross sectionand each control rod having a cross-shaped cross section,wherein a unitcell of a square-form is composed of two first fuel assemblies and twosecond fuel assemblies, said two first fuel assemblies obliquelyadjoining each other, a plurality of unit loading patterns arranged in acentral area of the initial core, each unit loading pattern beingcomposed of one unit cell and four control rods arranged at four cornersof said one unit cell, one of the second fuel assemblies, which has ahigher average enrichment than that of the other of the second fuelassemblies in each unit loading pattern, is divided into one side areanear to the control rod and another side area far from the control rodby a diagonal line, a number of fuel rods containing gadolinia in saidone side area being 2 or more than the number of fuel rods containinggadolinia in said another side area of the one of the second fuelassemblies which has a higher average enrichment than that of the otherof the second fuel assemblies in each unit loading pattern.
 9. Aninitial core claimed in claim 8, wherein said unit loading patterns arearranged to constitute control cells of a square-form, each control cellbeing composed of four fuel assemblies, said second fuel assemblieshaving a higher average enrichment being adjoined with each other.